1. Field of the Invention
This invention relates to short range communications using surface acoustic wave (SAW) expanders and compressors.
2. Description of the Prior Art
SAW technology is well known for its excellent radio frequency (RF) performance, low cost and small size. SAW is a passive thin film technology that does not require any bias current in order to function. SAW expanders and compressors have been used in RADAR applications for many years.
The basic xe2x80x9cbuilding blockxe2x80x9d of SAW expanders and compressors is the interdigital transducer (IDT) such as shown in FIG. 1. An IDT 10 is a series of thin metal strips or xe2x80x9cfingersxe2x80x9d 12 fabricated on a suitable piezoelectric substrate 14. One set of fingers is connected to an input/output terminal 16, while the opposite set of fingers is connected to another terminal 18. In single-ended IDTs, terminal 18 is grounded. For differential input signals however, terminal 18 is a pulse input/output terminal. Spacing xe2x80x9cWxe2x80x9d between IDT segments is adjusted to conform to the desired chip period of the coded sequence. When excited by a narrow pulse at terminal 16, the IDT generates a coded output SAW which propagates in both directions perpendicular to the fingers 12. If a similarly coded SAW impinges on the fingers 12, then an autocorrelation function is performed and a peak, with associated side lobes, is generated at terminal 16. These abilities of SAW expanders and compressors are well known in the prior art, having been demonstrated for example in Edmonson, Campbell and Yuen, xe2x80x9cStudy of SAW Pulse Compression using 5xc3x975 Barker Codes with Quadraphase IDT Geometriesxe2x80x9d, 1988 Ultrasonics Symposium Proceedings, Vol. 1, 2-5 October 1988, pp. 219-222.
Thus the structure shown in FIG. 1 can operate as both a SAW expander, generating a SAW output from a single pulse input, and a SAW compressor, generating a single pulse or peak output from a SAW input. Terminal 16, as well as terminal 18 in differential IDTs, is both a pulse input terminal and a pulse output terminal. Conversion of an output SAW into an electrical signal for further processing in conventional communications circuits and subsequent transmission through an antenna is accomplished by adding a transmit IDT 24, aligned with the IDT 22, as shown in FIG. 2. Both IDTs can be fabricated on the same substrate 14. A SAW output from IDT 22 is converted into an electrical signal by TX IDT 24. A SAW receiver would have the same structure as in FIG. 2. A signal input to a receive IDT from receiver processing circuitry would be converted to a SAW which is input to IDT 22. Like the IDT 22, the TX IDT 24 may be a differential IDT, wherein the grounded lower terminal would be a pulse output terminal.
The geometry of adjacent IDT fingers 12 is shown in FIG. 3, where Tf is the width of a metallized finger 12 and Ts is the width of the space between the fingers 12. In typical designs both Tf and Ts are equal to a quarter of a wavelength, xcex/4. Since wavelength is inversely proportional to frequency of operation, higher frequency IDTs require thinner fingers 12 located in close proximity to each other, which complicates fabrication and reduces fabrication yields. For example, for a typical SAW system operating in the Industrial, Scientific and Medical (ISM) band at 2.4 GHz the xcex/4 dimension could be in the order of 0.425 microns, depending upon the substrate chosen.
Previous communications system designs sought to overcome these manufacturing difficulties by using lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with mixers and local oscillators, as shown in FIG. 4. In the typical prior art communication system 30, the lower frequency 266 MHz signal generated by transmit IDT 20 is up-converted in mixer 34, which receives a 734 MHz signal from local oscillator 36. The output from mixer 34 is filtered in high pass filter 38 to produce a 1 GHz signal which is transmitted through antenna 40. On the receive side, the process is reversed in antenna 42, mixer 44, low pass filter 46 and receive compressor IDT 20xe2x80x2. As discussed above, transmit IDT 20 and receive IDT 20xe2x80x2 have similar structure. Undesirably, the mixers 34 and 44, oscillator 36 and filters 38 and 46 from the communications system 30, result in additional cost, power consumption, occupation in space and a much complex system than is desired for low-cost, low power, short range communication systems. Therefore, there remains a need in the art to reduce the number of components in such a communication system.
High-frequency communication techniques involving more conventional non-SAW based circuits and systems also exist. Bluetooth(trademark) wireless technology is one such prior art example. Bluetooth is a de facto standard, as well as a specification for small-form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable wireless devices. The current Bluetooth short range communications specification operates in the 2.4 GHz (ISM) band; however, in reality the technology for mobile communication devices involves undesirable high cost, substantial power consumption and relatively complex hardware.
It is an object of the present invention to overcome some of the drawbacks of the prior art.
It is also an object of the present invention to provide a low cost SAW-based communication method and system. As an illustrative example of the cost reduction, SAW devices utilized by the present invention for filtering at near-ISM band frequencies may cost approximately $1.00 each. In contrast, a comparable semiconductor Bluetooth solution may cost greater than $10.00.
It is a further object of the invention to provide SAW-based transmit and receive units which are easily manufactured. The manufacturing required for the present invention allows for SAW fabrication that utilizes simple, single layer photolithographic techniques.
Another object of the invention is to provide a low power SAW solution for short range communications. The SAW uses passive thin film technology and requires only a pulse to excite and produce an RF waveform. Likewise it can perform an autocorrelation function passively. This compares to prior SAW techniques which require frequency conversion circuitry such as mixers, filters and oscillators, and the complex Bluetooth techniques that require separate receive, transmit and processing circuitry. In mobile communication environments, power consumption and size are of primary importance.
A still further object of the invention is to provide a SAW-based communication arrangement which occupies minimal space. A complete SAW package in accordance with the invention is in the order of 3 mmxc3x973 mm.
The inventive SAW system reduces manufacturing complexity and cost and increases production yields by exploiting second harmonic components produced by expander/compressor IDTs. This allows the IDTs to be fabricated with larger finger widths than would be required according to known IDT methods and devices.
In the invention, a wireless communication system comprises an expander/compressor interdigital transducer (IDT) which produces a surface acoustic wave (SAW) output comprising frequency components at a fundamental frequency and a plurality of harmonic frequencies when excited with an electric input signal and produces an electric output signal when excited by a SAW input at the fundamental frequency or one or more of the plurality of harmonic frequencies, a transmit IDT positioned adjacent to the expander/compressor IDT and switchably connected to an antenna, and a receive IDT positioned adjacent to the expander/compressor IDT and switchably connected to the antenna, wherein the transmit IDT and the receive IDT are configured to operate at one of the harmonic frequencies.
In accordance with another aspect of the invention, a communication system comprises an expander IDT configured to produce a SAW output having a fundamental frequency and a plurality of harmonic frequencies when excited with an electric input signal, a transmit IDT positioned adjacent to the expander IDT and connected to an antenna, a receive IDT connected to the antenna; and a compressor IDT positioned adjacent to the receive IDT and configured to produce an electric output signal when excited by a SAW input comprising the fundamental frequency or one or more the plurality of harmonic frequencies, wherein the transmit IDT and the receive IDT are configured to operate at one of the harmonic frequencies.
The transmit IDT converts a SAW into an electric signal for transmission via the antenna and the receive IDT converts an electric signal received via the antenna into a SAW. The fundamental frequency may be 1.2 GHz and the transmit IDT and receive IDT operate at the second harmonic frequency of 2.4 GHz. Wireless communication systems according to the invention may be installed in both a wireless mobile communication device and a wireless earpiece detachable therefrom, to provide for communication between the mobile device and the earpiece. In a further embodiment of the invention, a SAW-based wireless communication system is installed in a wireless mobile communication device, a wireless earpiece detachable therefrom and a holder for the mobile device connected to a personal computer (PC), to provide for communication between the device and the PC through the holder, the device and the earpiece, and the earpiece and the PC through the holder.
The electric input and output signals associated with any of the IDTs may be either unbalanced or differential signals.
An expander/compressor IDT, or an expander IDT and a compressor IDT are preferably configured to embody a code and thereby produce a coded SAW output when excited with an input electric pulse and an output electric pulse when excited by a coded SAW input. The code embodied by these IDTs may be a Barker code such as a 5-bit Barker code, and may be used for example to represent identification information for an article with which the wireless communication system is associated.
According to a further aspect of the invention, a passive wireless communication system comprises an antenna for receiving communication signals and converting the received communication signals into electric antenna output signals and converting electric antenna input signals into output communication signals and transmitting the output communication signals, a first IDT connected to the antenna and configured to produce first SAW outputs in response to the electric antenna output signals and to produce the antenna input signals in response to first SAW inputs, a second IDT positioned adjacent to the first IDT and configured to produce a second SAW output comprising frequency components at a fundamental frequency and a plurality of harmonic frequencies when excited with an electric signal and to produce an electric signal output when excited by a second SAW input at the fundamental frequency or one or more of the plurality of harmonic frequencies, and a termination circuit connected to the second IDT, wherein the first IDT is configured to operate at one of the harmonic frequencies, the termination circuit causes the second IDT to reflect a second SAW output toward the first IDT in response to each first SAW output produced by the first IDT, and the first IDT produces an antenna input signal in response to each reflected second SAW output from the second IDT. The second IDT in such a passive system may be configured to embody a code.
In a particular embodiment of this aspect of the invention, a passive wireless communication system further comprises a third IDT which is positioned between the first IDT and the second IDT and reflects a third SAW output toward the first IDT in response to the first SAW output produced by the first IDT, wherein the first IDT produces a second antenna input signal in response to the reflected second SAW output from the third IDT. The passive wireless communication system may also include a fourth IDT which is positioned adjacent to the second IDT on a side of the second IDT opposite to the third IDT and reflects a third SAW output toward the first IDT in response to the first SAW output produced by the first IDT, wherein the first IDT produces a third antenna input signal in response to the reflected third SAW output from the fourth IDT. When the passive wireless communication system includes a third IDT and/or fourth IDT, these IDTs are preferably configured to operate at one of the harmonic frequencies, which may be the same as the harmonic frequency at which the first IDT operates.
The third and fourth IDTs, like the second IDT, may be connected to a respective termination circuit. A termination circuit is preferably either an open circuit, a short circuit or a sensor circuit. The passive wireless system preferably receives communication signals from a remote interrogation system, and through operation of the IDTs and termination circuit, automatically and passively responds to the remote interrogation system.
An IDT which may be used in embodiments of the invention preferably comprises a pair of substantially parallel electrically conductive rails and one or more groups of interdigital elements, each group comprising a plurality of interdigital elements. Each interdigital element is connected to one of the rails and extends substantially perpendicular thereto toward the other rail. Any such IDT may be configured to embody a particular code as determined by a connection pattern of the interdigital elements in each group. A coded IDT performs a passive autocorrelation function on a SAW input based on the code to thereby produce an electric pulse output when the SAW input is similarly coded.
A SAW system according to the invention may be employed in the design of virtually any new short range wireless communication system, for example to enable communication between an earpiece unit and an associated mobile communications device, as described briefly above and in further detail below. The inventive systems may also replace RF signal generation circuitry in existing short range communications system, including for example xe2x80x9cBluetoothxe2x80x9d systems. A further system in accordance with the invention may be employed in xe2x80x9csmartxe2x80x9d identification tag systems and remote interrogation systems such as inventory systems and meter reading/telemetry systems.
Further features of the invention will be described or will become apparent in the course of the following detailed description.